Musnad Abu Ya'la al-Mawsili 154:118

Abu Abdullah al-Nukri narrated to us, Abu Dawud narrated to us, Muhammad bin Mihran al-Qurashi narrated to us, he said: My grandfather, Abu al-Muthanna, narrated to me, from Ibn Umar, he said: The Messenger of Allah, may Allah bless him and grant him peace, used to pray four [rak'ahs] before the Asr [prayer].